Various types of devices are known that function to provide back-up power to computers when mains power fails, generally using energy stored in batteries. Such devices can be internal or external to the computer and are sometimes known as uninterruptible power supplies (UPSs) or battery back-up units (BBUs). Examples of these devices can be found described for instance in WO95/17777, GB2262170 and WO98/34314.
Typically, commercially available such UPS devices that are designed for use with modern personal computers are either in the form of a plug-in card for installation within the computer or a external device which is connectable in series with the power lead.
Usually, such devices alert the user to a power failure via some kind of on-screen message, the communication between the device and the computer being carried out via one of the computer's serial ports.
Whilst this arrangement is generally satisfactory it does present certain drawbacks in practice. First, the serial ports of a computer are a scarce resource and if one is taken up by an UPS device, then that port is not available for other connections. Second, the devices require a dedicated driver program to be developed to interface them with the operating system. Thirdly, the user may not notice or may be away from the computer when they are warned of a power failure.
Power management, and essentially power saving is becoming an important concern for computer manufacturers, inter alia for environmental reasons. Increasingly, modern personal computers include power management features that allow the computer to enter various power saving states. Generally, a modern personal computer has at least four different states with different patterns of power consumption.
In a first state, the computer is off and unplugged, and the power consumption is nil. In a second state, the computer is OFF but the power supply unit is connected to the mains power supply and the computer may be woken by a signal received via a computer network. Power supplies typically provide a low level (less than 5 watts) of auxilliary power in this state. A third state is of course the ON state, with the computer working normally.
In addition, at least one power consuming sleep or standby mode--a fourth state--is also generally provided. In such sleep modes, the functionalities of the computer are degraded, but the computer may be relatively rapidly restored to the ON state. In such sleep states, at least some data recording the state of the system when in entered the sleep state is generally stored in dynamic memory. For this reason, such states are sometimes referred to as `suspend to RAM`. Auxilliary power is also provided in these states.
The computer passes from the second state to the third state when the ON/OFF button of the computer is activated to switch the computer on. The computer passes from the third state to the fourth state in certain predefined conditions, for instance when it has been inactive for a certain programmed period of time. The computer passes from the fourth state--sleep mode--to the third state--ON--when the computer is subsequently woken up. This may for instance be the case when the user presses a key on the keyboard of the computer.
The trend in PC design, as expressed for example in the ONNOW initiative promoted by Microsoft Corp. is to arrange the system so that it is supplied with power for long periods of time, but is placed into a low power sleep or standby state when not in use, rather than being switched off. It is therefore to be expected that the chance of the PC being in some kind of sleep or standby state when a power failure occurs is quite high in practice.
EP-A-0805386 A1 proposes a system in which a power supply includes an internal power source. When the system is in a normal operating state or a lower power consumption standby state and external power is disrupted, the power supply is operative to generate a control signal to power management circuitry and switch to the internal power source. The power management circuitry causes the computer to change to a suspend state in which the entire state of the system is saved to a non-volatile storage device.
This invention is intended to mitigate the drawbacks of the above-described prior art by providing a convenient and low cost back-up power accessory for a computer which serves to enable the computer to be safely shutdown in the event of a power failure whatever its operating state at the time of the power failure and which has reduced impact on the manufacturing cost of the computer for which it is intended.